Disclaimer: I am not all that e-smart. Sometimes with sheer persistence, you get lucky... with a lot of help from your friends. In addition to the small 12v battery, there is another source of stored electrical energy in the Gen II at least: an ultra-cap (?) in the brake system. IIRC, it is a paper-back book sized black box mounted vertically next to the 12v battery. I don't know what voltage it runs on. Given the relatively small power needs and short time required to boot the appropriate cpu(s) to get the HV battery switched on and powering the DC/DC converter, would it be worth considering finding/hacking a way to momentarily tap into the ultra/cap? Yeah, great caution needed as it is an essential safety system.